Amphibious vessels or ships (hereinafter “ships”) for traveling over the surface of a body of water or over terrain which approximates a fluid such as muddy earth or wet sand, must provide for both flotation and propulsion. While many different techniques have been used throughout history to provide propulsion in ships, flotation has typically been provided in only one way. That is, according to Archimedes' principle, a ship (or any other floating object) must displace an amount of water that equals or exceeds its own weight. Fresh water weighs about 62.5 pounds per cubic foot, and a significant portion of the ship must typically be submerged to provide the required flotation.
The propulsion system employed in a ship must overcome forces that resist the ship's movement over the water. Both the air, acting against the portion of the ship above the water-line (hereinafter “super-structure”), and the water, acting against the portion of the ship submerged below the water-line (hereinafter “hull”) resist movement of the ship; however, the contribution of the water is by the far the most significant.
The water resists movement of the hull through the water for two reasons. First, water is viscous as a result of its molecules being somewhat attracted to one another. This viscosity results in a frictional force that resists shearing of the water. This frictional force increases in proportion to the velocity or speed of the ship.
A second form of resistance to movement of the hull through water results from the fact that the water has inertia, and so resists accelerations. Displacing a given volume of water requires accelerating the volume of water sufficiently quickly that the water can be moved out of the way of the ship. Producing this acceleration requires a force.
The propulsion system must provide sufficient propulsive force that both of these reactive forces applied to the hull by the water are overcome. As the ship travels faster, both of these reactive forces increase, and the speed of most ships is severely limited as a result of the mechanical requirements on the propulsion system, as well as the requirement to supply the propulsion system with sufficient fuel, to overcome these forces. Further, there is a theoretical maximum speed determined for a ship employing a displacement hull, i.e., a hull that remains submerged to the same extent regardless of ship speed, that is proportional to the square root of the length of the hull.
The fact that the water reacts against the submerged hull and thereby limits the speed of a ship has led to the use of the planing hull or hydrofoil which lifts out of the water as ship speed is increased. An extreme case of minimizing the ship's interaction with the water is found in the hovercraft, where no part of the ship is submerged, so that there is in essence no hull. However, both hydrofoils and hovercraft have high fuel requirements and find limited use. They are not generally practical for use in large ships requiring a high carrying capacity.
There is a need in the military to employ large ships that can carry troops and equipment to remote parts of the globe with the utmost speed. It is also more generally desirable to improve ship efficiency by maximizing speed and minimizing fuel requirements. Accordingly, there is a need for a ship employing a buoyant propulsion system as described herein.